Transducer And System Dependency Of Scanning Acoustic Microscope Images For Plastic Encapsulated Microelectronics

Abstract: Scanning Acoustic Microscope (SAM) has been evaluated for its capability and consistency in imaging plastic encapsulated microelectronics (PEMS) components. SAM has been used in the past as a tool to observe or measure package interfacial delaminations during and after accelerated environmental stress tests. We have been interested in establishing the SAM technique as a repeatable and reproducible method for contractor evaluation of PEMs reliability under temperature cycling and temperature/humidity stresses.

In the process of developing SAM contractor specifications, we have imaged selected PEMS components with SAM systems made by two different manufacturers. They are AD795 Si operational amplifiers and MMBT2222 NPN and MMBT2907 PNP Si bipolar transistors. Both SAM manufacturers use the same 15, 25, 30, 35, 50, and 75 MHz frequency ultrasonic transducers. With either system, the SAM reflection images for the same PEMs part and the same transducer are quite similar, but the delamination information produced by each system is different. Our studies indicate that the machine delamination information given by the built-in-phase inversion algorithms is system, transducer, and operator dependent. Thus, the delamination information given by the built-in-phase inversion algorithms requires careful interpretation to avoid either false positives or negatives. An experienced in-house SAM evaluation may be necessary to verify delaminations reported by an outside testing laboratory and any recommendation for a follow-up cross-section of the imaged parts.

For now, we consider SAM as a secondary tool during characterization and reliability test processes. No reject and accept criteria for a PEMs packaged part exists other than IPC/JEDEC J-STD-020A1. Delamination is frequently observed on the mold compound to the lead frame (die paddle) interface after an environmental test. Although this type of delamination is not listed as a “failure” in J-STD- 020A, we consider it to be a warning sign.